1. Field of the Invention
The present invention generally relates to improvements in a cellular mobile communications system and, more specifically, to a method and arrangement for controlling handoff in a cellular mobile communications system. The present invention is characterized in that the number of handoff requests can effectively be reduced as compared with known techniques.
2. Description of Related Art
It is well known in the art to apply frequency reuse in a mobile communications system in order to satisfy various objectives such as large subscriber capacity, efficient spectrum use, widespread availability, adaptability to traffic density, etc.
Frequency reuse refers to the use of the same radio carrier frequency in a number of different channels to cover different areas which are separated from one another by distances sufficient that objectionable cochannel interference does not occur. The so called cellular mobile communications system utilizes such a concept.
When a mobile unit, during a call, moves out of the service coverage of a particular land site, the signal strength measured at the particular land site falls below a handoff threshold level. Thus, the present land site requests an MTSO (Mobile Telephone Switching Office) to implement a handoff or switch over the mobile unit to another suitable nearby land site. The MTSO, in response to the handoff request, instructs the neighboring land sites to measure the strength of the signal transmitted from the mobile unit to be handed off.
After each of the nearby land sites completes measuring the signal strength of the mobile unit to be handed off, it informs the MTSO of the results. The MTSO checks the results which are received and then implements a handoff if there is a land site to which the mobile unit can be suitably handed off. Otherwise, the handoff is not carried out.
Accordingly, if a handoff is not carried out and if the signal strength measured at the land site with which the mobile unit has established a call, is still below the handoff threshold, then the land site again generates a handoff request to the MTSO after a predetermined time interval. If these conditions continue, the land site repeatedly sends handoff requests to the MTSO at the above-mentioned predetermined time interval. This time interval has been selected to meet severe conditions as when a mobile unit moves at a high speed from cell to cell without cutting off transmission.
The above-mentioned related art however, suffers from the following problems. For example, a wastefully large number of handoff requests is transmitted when a mobile unit moves at a low speed in the vicinity of a cell boundary. Since the time interval of issuing the requests is fixed to a short term as mentioned above, handoff requests are frequently sent to the MTSO although a handoff is not or can not be achieved.
Further, in the case where a hand-held mobile unit is used in a building or the like, the strength of the signal transmitted to a land site from the mobile unit is likely to be below the handoff threshold even though the building is located in a good service area. In this instance, the land site detects the signal strength being below the handoff threshold and thus sends handoff requests to the MTSO at the predetermined time intervals irrespective of the fact that the hand-held mobile unit will almost certainly not be handed off to any other land site. The generation of such frequent handoff requests from the hand-held mobile units at short time intervals, induces serious saturation problems (for example) as the number of subscribers using hand-held mobile units has greatly increased in recent years.
Still further, in order to meet the ever increasing number of subscribers, subdividing each of the cells into smaller ones tends to greatly increase the total number of handoff requests. Accordingly, each of the land sites is undesirably required to be equipped with sophisticated and expensive instruments for measuring signal strength in response to the requests from the MTSO. In addition to this, the MTSO is also required to handle an ever larger number of handoff requests.
It is therefore highly desirable to reduce the number of handoff requests in order to lessen the burden on the land sites as well as the MTSO in terms of the number of handoff operations and the complex hardware requirements associated therewith.